Given, final temperature T = ??
MILk m1 = 200 ml = 0.2 kg
tempe t1 = 90T\;=\;\frac{18.15}{0.21}
for water, m2 = 10 ml = 0.01 kg
Temperature t2 = 15°C
By using the formula that we already know,
\(T\;=\;\frac{m1st1\;+\;m2s2t2}{m1s1+m2s2}\)
\(T\;=\;\frac{18.15}{0.21}\)
T = 86.42 °C